Johann Sebastian Bach composed the church cantata O Ewigkeit, du Donnerwort (O eternity, you word of thunder),[1] BWV 20, in Leipzig for the first Sunday after Trinity, which fell on 11 June in 1724.

As usual for Bach's chorale cantatas to come in the cycle, selected hymn stanzas were retained while the others were paraphrased by a contemporary poet who transformed their ideas into a sequence of alternating recitatives and arias.

Bach scored the cantata for three vocal soloists, a four-part choir and a Baroque instrumental ensemble of two tromba da tirarsi, three oboes, two violins, viola and basso continuo.

All instruments play in the opening chorale fantasia, in which the soprano sings the hymn tune as a cantus firmus.

The text is based on Johann Rist's hymn "O Ewigkeit, du Donnerwort", which was published in the collection Himlische Lieder (Heavenly songs) in Lüneburg in 1642.

Overall, the poet stayed close to the hymn's text, which is characteristic for the early cantatas in Bach's second annual cycle.

[11] The poet was possibly Andreas Stübel, who died in 1725, which would explain why Bach did not complete the full cycle, but ended on Palm Sunday.

[7] The chorale theme was composed by Johann Schop for the hymn "Wach auf, mein Geist, erhebe dich", which appeared in the collection Himlische Lieder.

Bach scored the work festively for three vocal soloists (alto (A), tenor (T) and bass (B)), a four-part choir (SATB), and a Baroque instrumental ensemble: tromba da tirarsi (slide trumpet, Tt), three oboes (Ob), two violins (Vl), viola (Va), and basso continuo (Bc).

The recitatives are mostly secco, with an arioso only in movement 9 on the words "Pracht, Hoffart, Reichtum, Ehr, und Geld" (splendor, pride, riches, honor, and wealth)[1] from the chorale.

[16] Gardiner notes the following about the first pair of recitative and aria: The tenor prolongs the mood of torment ... ramming home the themes of anxiety, pain, hell and the quaking heart.

[17] The first motif in movement 10 is sung by the two singers of the duet on the words O Menschenkind ("o child of man") and are repeated instrumentally as a hint of that warning.

[17] Both parts of the cantata are concluded by the same four-part chorale setting,[18] asking finally "Nimm du mich, wenn es dir gefällt, Herr Jesu, in dein Freudenzelt!"

[19] Ensembles playing on period instruments in historically informed performance and a choir of one voice per part (OVPP) are marked by green background.